Color liquid crystal display projectors generate display images and project them onto display screens, typically for viewing by multiple persons or viewers. The display images may be formed by transmitting light from a high-intensity source of polychromatic or white light through or reflected from an image-forming medium such as a liquid crystal display (LCD).
FIG. 1 is a schematic diagram of a prior art multi-path reflective color liquid crystal display projection system 10 that utilizes color separating mirrors 12R, 12BG, and 12G in combination with polarization selective polarizing beam splitters 14R, 14G, and 14B and reflective liquid crystal displays 16R, 16G, and 16B to provide a high resolution, high brightness display.
Projection system 10 includes a light source 18 that directs white light through a polarizer (or polarization converter) 20 that provides polarized light to a pair of crossed dichroic mirrors 12R and 12BG. Dichroic mirror 12R reflects red light components along a red optical path 22R that is folded by an achromatic fold mirror 24R. Green and blue light passes through mirror 12R. Mirror 12BG reflects blue and green light components along a blue-green optical path 22BG that is folded by an achromatic fold mirror 24BG. Red light passes through mirror 12BG. Mirror 12G reflects green light components along a green optical path 22G and allows the blue light components to propagate along a blue optical path 22B. As a result, mirrors 12R, 12BG, and 12G cooperate to separate polarized red, green and blue light components and deliver them to polarizing beam splitters 14R, 14G, and 14B.
Each polarizing beam splitter 14 includes a pair of right-angle prisms having their inclined faces positioned against each other with a polarization selective dielectric film (not shown) positioned therebetween. As is conventional for polarizing beam splitters, P-polarized light passes through the dielectric film and S-polarized light is reflected. S- and P-polarizations are conventional nomenclature referring to a pair of orthogonal linear polarization states in which, with regard to a polarization selective dielectric film, S-polarized light can be said to "glance" off the film and P-polarized light can be said to "pierce" the film. Polarizer 20 transmits the red, green and blue light components as predominantly S-polarized light, so nearly all the light received by polarizing beam splitters 14R, 14G, and 14B is reflected by the dielectric films to reflective liquid crystal displays 16R, 16G, and 16B.
In one implementation, reflective liquid crystal displays 16 are quarter wave-tuned (i.e., with 45.degree.-60.degree. twists) twisted nematic cells and reflect light from each pixel with a polarization that varies according to the control voltage applied to the pixel. For example, when no control voltage is applied (i.e., the pixel is in its relaxed state), the pixel imparts maximum (i.e., a quarter wave) phase retardation that results in a polarization rotation for suitably aligned polarized light. Each pixel imparts decreasing polarization rotation with increasing control voltage magnitudes until the pixel imparts no rotation (i.e., the pixel is isotropic).
In the relaxed state of a pixel, the polarization state is reversed when the light is reflected, so that the S-polarized light becomes P-polarized light. The P-polarized light then passes through the dielectric film of the polarizing beam splitter toward a crossed-combining prism 26 (also known as an X-cube) to be incorporated into the display image. With non-zero control voltages, the pixel reflects the light with corresponding proportions of P- and S-polarizations. Control voltages of greater magnitudes in this example cause greater portions of the light to be reflected with S-polarization, with all the reflected light having S-polarization at the greatest control voltage. The portion of the light with S-polarization is reflected by the dielectric films in polarizing beam splitters 14 back toward the illumination source and are not incorporated into the display image.
Such a multi-path reflective color liquid crystal display projection system 10 can provide improved imaging characteristics over more conventional projections systems that use transmissive liquid crystal displays. Reflective liquid crystal displays do not suffer from the low transmissivity characteristic of transmissive displays, and hence the relatively low brightness of their projection systems. Moreover, the reflective liquid crystal displays are relatively easier to fabricate and miniaturize than conventional transmissive liquid crystal displays, which can allow lower production costs and smaller, more portable projection systems.
While it may have advantages over conventional transmissive projection systems, such a multi-path reflective color liquid crystal display projection system 10 suffers from disadvantages that impair its imaging characteristics. One of crossed mirrors 12R and 12BG is actually formed with two mirror halves that are positioned behind and in front of the other of mirrors 12R and 12BG. Proper alignment of the mirror halves is very difficult and rarely achieved. As a consequence, the images reflected by the mirror halves are mis-aligned, which can result in readily discernible mis-alignments in the image halves. The relatively common misalignment between the mirror halves introduces, therefore, generally unacceptable image errors that may appear as ce-coupled image halves that are improperly joined along an apparent seam.
One implementation of a multi-path reflective color liquid crystal display projection system according to the present invention utilizes two color separating mirrors in combination with two polarizing beam splitters and reflective liquid crystal displays to provide a high resolution, high brightness display. The projection system includes a light source that directs white light through a polarization converter that provides S-polarized light to a first angled dichroic mirror. In one implementation, the dichroic mirror reflects two color components (e.g., green and one of the red and blue components) and passes one color component (e.g., the other of the red and blue components). The dichroic mirror provides a one-to-two color separation in which the green light component is reflected with one other light component.
A quarter wave plate and an achromatic mirror are positioned behind and parallel to the dichroic mirror and cooperate to convert the light that passes through the dichroic mirror (e.g., red light) from S-polarization to P-polarization. The P-polarized red light then passes through the dichroic mirror along the same optical path as the S-polarized green and blue color components.
A second angled dichroic mirror directs a selected one of the red blue components (e.g., blue) to a polarizing beam splitter that includes a pair of right-angle prisms having their respective inclined faces positioned against each other with a dielectric film therebetween. The dielectric film in the polarizing beam splitter is polarization-selective and may be achromatic or color-tuned. With a color-tuned dielectric film, the polarizing beam splitter transmits all color components of light other than the selected component (e.g., blue), regardless of polarization, while functioning as a conventional polarizing beam splitter for the selected color (e.g., blue light). Accordingly, the polarizing beam splitter reflects S-polarized blue light toward a reflective liquid crystal display, and any P-polarized blue light passes out of the polarizing beam splitter with the non-blue light (i.e. red or green light).
The remaining color components (e.g., red and green) pass through the second angled dichroic mirror to a second polarizing beam splitter having a pair of right-angle prisms with their respective inclined faces positioned against each other with a dielectric film therebetween. A reflective liquid crystal display is positioned in alignment each of two adjacent sides of the second polarizing beam splitter.
The two color components (e.g., red and green) received at the second polarizing beam splitter are of respective S- and P-polarizations. The S-polarized green light is reflected by the dielectric film to one of the reflective liquid crystal displays, and the P-polarized red light passes through the dielectric film to the other of the reflective liquid crystal displays. The images formed at the three reflective liquid crystal displays are reflected to and combined by a simple combiner formed by a pair of right angle prisms.
The reflective liquid crystal displays are quarter wave-tuned twisted nematic cells and reflect light from each pixel with a polarization that varies according to the control voltage applied to the pixel. For example, the pixel in its relaxed state may have quarter wave retardation and maximum polarization rotation and may have decreasing polarization rotation with increasing control voltage magnitudes up to a maximum control voltage magnitude at which the pixel is isotropic and imparts no polarization rotation.
A second implementation of a multi-path reflective color liquid crystal display projection system according to the present invention utilizes one color separating mirror in combination with polarizing beam splitters and reflective liquid crystal displays to provide a high resolution, high brightness display. This projection system also includes a light source that directs white light through a polarization converter that provides S-polarized light to an angled dichroic mirror. In one implementation, the dichroic mirror reflects two color components (e.g., green and one of the red and blue components) and passes one color component (e.g., the other of the red and blue components).
In this implementation, however, the two reflected color components pass first through a green-tuned polarizing beam splitter. Each of the polarizing beam splitters includes a pair of right-angle prisms having their respective inclined faces positioned against each other with a dielectric film therebetween. Dielectric film in the green-tuned polarizing beam splitter is polarization-selective and functions to transmit all color component light other than green, regardless of polarization, while polarizing beam splitter functions as a conventional polarizing beam splitter for green light. Accordingly, the green-tuned polarizing beam splitter reflects S-polarized green light toward a reflective liquid crystal display, and any P-polarized green light passes with the non-green light (i.e. red or blue light) toward an achromatic fold mirror. The remaining two polarizing beam splitters each receive one of the red and blue light components with an S-polarization. These polarizing beam splitters reflect the S-polarized red or blue light toward respective reflective liquid crystal displays.
The multi-path reflective color liquid crystal display projection systems of this invention provides improved imaging characteristics over more conventional projection systems that use transmissive liquid crystal displays. Reflective liquid crystal displays do not suffer from the low aperture ratios characteristic of transmissive displays, and hence the relatively low brightness of their projection systems. Moreover, the reflective liquid crystal displays are relatively easier to fabricate and miniaturize than conventional transmissive liquid crystal displays, which can allow lower production costs and smaller, more portable projection systems.
In addition, this multi-path reflective color liquid crystal display projection system includes fewer optical elements and overcomes poor imaging characteristics of prior multi-path reflective color liquid crystal display projection systems (e.g., FIG. 1). This projection system eliminates one of the three polarizing beam splitters and the "crossed" dichroic mirrors. The crossed mirrors are actually formed with three mirrors, one whole mirror and two mirror halves that are positioned behind and in front of the whole mirror. Proper alignment of the mirror halves is very difficult and rarely achieved. As a consequence, the images reflected by the mirror halves are mis-aligned, which can result in readily discernible mis-alignments in the image halves. Elimination of the crossed mirrors prevents the misalignment between image halves characteristic of imaging systems with crossed mirrors.
Furthermore, this projection system provides increased compactness and simplicity over prior reflective color liquid crystal display projection systems by incorporating color separating functionality into the green polarizing beam splitter rather than having a pair of crossed dichroic mirrors. This integrated functionality in polarizing beam splitter allows elimination of two of the three dichroic mirrors and one of the two achromatic fold mirrors in prior projection systems. Elimination of these components reduces the overall cost of projector system and provides a shortened optical system that allows projector system to be even smaller and more portable than prior systems.
Additional objects and advantages of the present invention will be apparent from the detailed description of the preferred embodiment thereof, which proceeds with reference to the accompanying drawings.